Aging of High Temperature Insulation Systems with Alternative Fluids

Jean-Claude Duart DuPont International Sàrl

Geneva, Switzerland Jean-Claude.Duart@che.dupont.com

Lisa C. Bates E. I. du Pont de Nemours and Co.

Richmond, VA, USA Lisa.C.Bates@usa.dupont.com

Abstract— For many years traditional design of liquid-immersed transformers has been based on the combination of cellulose solid insulation and mineral oil. As a result, average temperature rise for windings is normally limited to 65 K, and top oil rise to 60 K as given in IEC 60076-2. Sometimes space or weight limitations or high ambient temperatures force designers to look for new solutions such as those with higher temperature rises using high temperature materials. Using aramid paper with a much higher thermal capability than Kraft cellulose paper can allow operating the equipment at a higher winding temperature rise without any negative impact on the insulation life.

Aramid solid insulation can be combined with either mineral oil or alternative fluids, like silicones or esters. Silicone and ester fluids are also materials with higher temperature class than mineral oil. Therefore combining them with aramid solid insulation gives a chance of not only operating windings at higher temperature but entire insulation systems to be more effective in terms of thermal properties. Using high temperature insulation materials allows for significant reduction of cooling system. Allowing higher temperatures within windings results in the possibility of applying higher current densities in winding conductors. This leads straight to significant savings in raw materials. Smaller cooling system, less conductor, and use of natural ester fluids could mean less environmental footprint of the equipment built.

In previous papers we presented thermal evaluation results of two alternative fluids, silicone and synthetic ester. In this paper we will also present findings of a natural ester fluid and the behavior of aramid solid insulation in these fluids at a range of temperatures.

Keywords-aging, aramid, ester, silicone, fluid

I. INTRODUCTION The use of alternative fluids in transformers has increased

since the beginning of this decade. In particular, distribution transformers with such fluids are being developed as they need to meet new stringent regulations on fire safety and higher environmental end-user requirements. Due to their improved thermal capabilities versus conventional fluids like mineral oil, they are also combined with advanced solid materials in insulation systems of liquid-immersed transformers. They operate beyond the typical top oil temperatures as given in

IEC 60076-2 [1]. New guidelines have been developed that provide the end users and OEM guidance on possible operating temperatures [2].

However it is becoming critical to characterize the long-term behavior of those fluids in combination with advanced solid materials. Current alternative fluids include silicone, synthetic esters, and natural ester. Understanding the impact of time and temperature on the properties will allow better anticipation of lifetime. Typical aging testing in sealed tube is being used to generally determine the compatibility of solid and liquid insulating materials. However, in the study described in this paper the environment conditions have been modified to allow better representation of the inside of a transformer. Particular attention has been given to the interaction with oxygen. Three conditions have been used: open with unlimited access of oxygen, sealed with an air blanket and sealed with a nitrogen blanket. We know that oxygen is a critical factor of the aging of fluids and methodologies have been developed to specifically study oxidation stability of electrical fluids [3, 4]. Mineral oil has not been tested in the current study as the aim of the work was to look at aging in temperatures where mineral oil would not be able to operate. The range of aging temperature selected for the study was 140°C to 170°C. It has also being shown in previous work that different alternative fluids have different oxidation behaviors [5, 6]. To characterize fluids after aging we have decided to use known methodologies (e.g., dielectric breakdown, acid number, viscosity, color) including dissolved gas analysis (DGA). However DGA measurements will not be presented in this paper.

In addition to thermal stability and lifetime of alternative fluids, it is important to evaluate their compatibility with new solid insulation materials. Recently a new high temperature resistant paper has been developed for liquid-immersed transformers. This new aramid paper, while expected to be resistant to temperatures beyond 130°C, has been evaluated and compared to current commercial aramid papers used as conductor insulation. This technical paper will present data on thermal aging of both aramid papers in various alternative fluids.

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II. BACKGROUND

A. Application of High Temperature Insulation Systems High temperature insulation systems were developed many

years ago combining aramid papers and aramid board with synthetic fluids [5]. Traction transformers for high speed trains are one of the initial applications that used these advanced insulation systems. In such applications the need to reduce weight and gain space led to the development of transformers having to operate at winding temperatures beyond those generally accepted for cellulose based insulation. More recently similar insulation systems have been studied for adaptation in distribution transformers. Building on the values that were already realized in traction transformers like the weight and size reduction, overloading or peak loading capability, manufacturers of distribution transformers also saw the value for higher reliability and lowest total owning cost.

The use of natural ester fluids has also increased in recent years. While the application of combining this type of fluid with high temperature solid insulation is still limited, it is believed that these systems will become an important option to transformer end-users in the near future.

B. Thermal Testing Methodology In order to implement high temperature insulation systems

it is important to understand the characteristics of both the solid insulation and fluid at temperatures above those typically seen in cellulose/mineral oil insulation systems. Thermal evaluation methods for materials used in liquid-immersed transformer applications have included both single temperature [7, 8] and dual-temperature models [9, 10]. In single temperature testing, all materials are held at one temperature. However, in a dual-temperature test, the fluid and solid material can be aged at different temperatures which would be more representative of the application in a transformer.

Our investigation of aging high temperature insulation with alternative fluids has focused on single temperature aging to study the fluid aging characteristics and has been previously described [5]. In single temperature aging, all materials were in a stainless steel vessel (Figure 1) and placed in an oven for a set period of time (500 – 2160 hrs). Variables included aging temperature, aging time, headspace environment, and whether the vessel was vented (Table I). Each test vessel contained

Figure 1. Single temperature vessels used for aging. Cell on left used for “open to air” and “closed with air”. Cell on right used with closed with N2

condition.

TABLE I. TEST CONDITIONS

Fluid Open to Air

Closed with Air

Closed with N2

Temp (°C)

synthetic ester 140, 150, 160

natural ester 140, 150, 160

silicone ----- ----- 150, 160, 170

fluid, aramid insulation (paper and board), copper, and steel. Fluid properties such as acid number, viscosity, power factor, color, and dissolved gases were measured for each condition. Tensile properties were measured for the aramid paper insulation. The board insulation was only present in the vessel to represent a realistic ratio between volume of solid insulation versus volume of fluid.

In the “open to air” condition, two holes in the test vessel lid were left open, allowing free exchange of gases in the cell with air in the oven. In the “closed with air” condition, both holes in the lid were closed, thus not permitting a free exchange of gasses. In both “open to air” and “closed with air” cases, the fluid filled approximately 1/3 of the volume of the vessel and the remainder was filled with air as a headspace. The volume ratio between headspace and fluid is much larger than would be found in a transformer. However, the “open to air” condition could be viewed as the worst case (e.g., most susceptible to oxidative degradation) while the “closed with air” (but with a large air volume) is a lesser case. In the “under N2” condition, a smaller aging vessel was used which was able to maintain a tight seal. The test case of aging under nitrogen would be the best case in terms of reduced oxidative degradation. It was critical in that case to design a new test vessel with seals that would guarantee no introduction of oxygen during the testing.

III. THERMAL AGING RESULTS

A. Aramid Insulation Figure 2 shows the tensile strength of two aramid papers

aged in all three fluids at the highest temperature, longest time, and most oxidizing aging condition that we studied. A new aramid paper which was recently developed and introduced specifically for application in liquid-immersed transformers was compared to current aramid papers used in similar applications. Neither aramid paper showed any significant change in tensile strength with aging in the synthetic ester, natural ester, or silicone fluid at any of the conditions. This is consistent with past testing which has shown the aramid insulation is a high temperature insulation [9, 11] and is compatible with any dielectric fluid.

Figure 2. Tensile strength – aged open to air for 2160 hrs. Error bars represent +/- 2 standard deviations. A = synthetic ester, B = natural ester, C =

silicone.

B. Silicone Fluid Results for silicone fluid testing were published previously

[5] and properties were stable within the test range maximum of 170°C when aged “open to air”. For example, the maximum acid number measured on the aged oil was 0.01 mg KOH/g (Figure 3) which is the “test limit for service-aged silicone fluid” in IEEE C57.111, “Guide for Acceptance of Silicone Insulating Fluid” [12]. Silicone fluids do not produce acidic components during oxidation however an increase in acid number could indicate degradation of other solid insulation materials or contaminants in the oil. Dielectric breakdown was above the guideline of 25kV (per ASTM D877 [13]) although the guide is based on disk electrodes and our measurement utilized partially spherical electrodes (IEC 60156 [14]) – both with the same gap between the electrodes. We are unable to compare viscosity values because the IEEE guide values are for measurements at 25°C whereas our measurements were made at 40°C.

The stability of the fluid in our thermal aging test is consistent with information reported by Dow Corning regarding their silicone fluid: “thermal breakdown of silicone fluid begins at temperature above 230°C…oxidation of silicone fluid will take place very slowly (in the presence of oxygen) at temperatures above 175°C” [15]. No further aging studies were performed using the “closed with air” and “under N2”

Figure 3. Acid number (ASTM D974) – open to air. A = synthetic ester, B = natural ester, C = silicone.

conditions because the properties of the fluid under “open to air” condition (considered as the most aggressive) did not show significant changes in the fluid. If testing under aging temperatures beyond 170oC was to be considered, the influence of oxygen in “closed with air” conditions should be then considered.

C. Synthetic and Natural Ester Fluids Dielectric breakdown (per IEC 60156, 2.5mm gap) of the

natural ester and synthetic ester fluids remains fairly stable with thermal aging up to 2160 hrs at 160°C when aged under all three headspace conditions with dielectric strength values of 19.2-38.4 kV/mm and 20.8-38.4 kV/mm for synthetic ester and natural ester fluids, respectively. IEC 61203 [16] suggests a dielectric strength of 12kV/mm for used synthetic organic ester fluids is sufficient for continued use of 35kV or lower equipment. IEEE C57.147 [17] “suggested limit of continued use of service aged natural ester fluids” is in the range of 20-25 kV/mm for testing with a 2 mm gap. Borsi had also found the breakdown strength of a synthetic ester fluid was stable after aging at 150°C for 1000 hrs while open to air [18].

Acid number of both ester fluids increases with time and temperature when thermally aged with a fixed air headspace or with an inert atmosphere (Figures 4-5). The natural ester fluid shows a large increase in acid number when aged under open air headspace (Figure 3). Only the acid number of the natural

Figure 4. Acid number (ASTM D974) – closed with air. A = synthetic ester, B = natural ester.

Figure 5. Acid number (ASTM D974) – closed with N2. A = synthetic ester, B = natural ester.

ester aged open with air exceeds 2 mg KOH/g which is the value cited in IEC 61203 as “satisfactory condition for continued use…for 35 kV or lower voltages equipment”. However, the provisional recommendation shown in IEEE C57.147 Table B.5 is to take action when the acid number exceeds 0.5 mg KOH/g. Our aging is likely more severe than typical operation in that our lowest temperature studied is 140°C. Even when aged under N2 (Figure 5) our acid numbers are generally higher than 0.3 mg KOH/g after 1125 hrs. The increase in acid number under these different headspace conditions (Figure 6) highlights the impact of oxygen exposure to the fluid and is consistent with recommendations in the recent IEEE standard on natural ester fluids to use natural ester fluids in closed systems [17].

Power factor of both the natural and synthetic ester fluids increases when the fluid is aged open to air (Figure 7) with a much larger increase observed in the synthetic ester fluid. Once air is limited, the power factor of both the natural and synthetic ester fluids seems more stable (Figure 8). Power factor recommendation of IEC 61203 [16] for synthetic organic ester fluids is <1% while IEEE C57.147 suggests power factor less than 3% are acceptable. However we cannot compare to our measured values because our measurement was made at 90°C while both these standards use values measured at room

Figure 6. Acid number (ASTM D974) after aging at 160°C for 2160 hrs under different headspace environments. A = synthetic ester, B = natural ester.

Figure 7. Power factor measured at 90°C (ASTM D924) – open to air. A = synthetic ester, B = natural ester, C = silicone.

Figure 8. Power factor measured at 90°C (ASTM D924) after aging at 160°C for 2160 hrs under different headspace environments. A = synthetic ester, B =

natural ester.

temperature. The higher power factor of the synthetic ester cannot merely be linked to the acid number because the natural ester fluid had higher acid number when aged open to air but has a lower power factor than the synthetic ester fluid under the same headspace condition. This points to the fact that the two ester fluids do behave differently and may need separate interpretation of aging results after years of service in transformers.

Viscosity of the natural ester fluid increases with time and temperature when aged “open to air” (Figure 9) while the synthetic ester fluid shows fairly stable viscosity. However, when aged closed with air or under inert atmosphere (Figure 10), the viscosity of both natural and synthetic ester fluids are relatively stable over the aging conditions.

Color of the synthetic ester fluid and natural ester fluid darkens with time and temperature when the fluid is aged with exposure to air. However, the color of both fluids are more stable when oxygen is limited or when aged under inert atmosphere. Changes in color could indicate a faster degradation of the fluid.

Figure 9. Kinematic viscosity at 40°C (ASTM D445) – open to air. A = synthetic ester, B = natural ester, C = silicone.

Figure 10. Kinematic viscosity at 40°C (ASTM D445) after aging at 160°C for 2160 under 2 headspace conditions. A = synthetic ester, B = natural ester.

IV. DISCUSSION OF RESULTS The single temperature type of test results presented here

show that our test is probably harsher than what an actual transformer would experience. For example, acid numbers are higher than current guidelines for equipment in use. This was expected and it leads us to examine dual temperature aging tests in which oil is kept at a lower temperature than the conductor similar to what can be observed in a transformer. However the choice of aging times (up to 2160 hr) and temperature scale (from 140 to 170oC) was critical to differentiate aging behavior of the fluids under the three environments. It is important to extend the aging times in order to better characterize the behavior of any insulating material particularly in the case of fluids.

Moisture was not taken into consideration. While the samples were dried and vacuum impregnated, additional moisture was not introduced into the cells to simulate a humid environment. Moisture parameter will be considered for future testing. We know that ester fluids do have a higher moisture saturation level. However it will be important to explore over time what is the role of moisture in the fluid.

Two different aging cells were utilized due to the difficulty in maintaining an air tight seal in the original cell. As a result of using a smaller cell for the “under N2” headspace, a smaller headspace was utilized for this condition versus the “open to air” and “closed with air” conditions. However, this difference in oil volume to headspace should not impact the test because the test is comparing an oxidative environment to a non-oxidative (but smaller) environment.

V. CONCLUSIONS Despite the property changes observed in both natural ester

and synthetic ester fluids, the dielectric breakdown voltage appears stable within the tested temperature and time range. Behavior of natural and synthetic ester fluids do differ as we noted the lack of correlation of acid number and power factor for both fluids after aging. The use of three conditions has allowed us to confirm the influence of the presence of oxygen as a critical parameter in the speed of degradation. Acid

number, power factor and viscosity are clearly showing the impact of the presence of oxygen on the aging of esters fluids. These three properties remain critical to evaluate the aging of fluids.

While the single temperature testing is less representative of the true behavior in a transformer it a powerful tool to understand the influence of various parameters like aging time, aging temperature, and oxygen presence. It also allows rapid verification of compatibility of any solid insulation material with a given fluid. In our case we could demonstrate that a new aramid paper structure was suitable for use in fluid filled environment and has a similar thermal capability as current commercial aramid papers.

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